In a crawler vehicle such as a power shovel, a crawler is driven using a sprocket fixed in an outer circumference of a rotational casing. Since the rotational casing is located in an outer side of a vehicle body, the rotational casing is rotated in a muddy state when the crawler vehicle travels on miry mud in a construction field.
In JP 2001-248735 A, published by The Japan Patent Office, proposes a seal structure for preventing a foreign object such as mud from invading a gap between such a rotational casing and a fixed housing fixed to the vehicle body. The seal structure has a labyrinth seal and a floating seal between the fixed housing and the rotational casing.
The labyrinth seal has a clearance having a cranked cross-sectional shape defined around the floating seal in order to prevent a foreign object such as mud from approaching the floating seal.
The floating seal has a pair of metal seal rings supported by an O-ring between the fixed housing and an inner wall surface of the rotational casing. The seal ring elastically supported by the O-ring installed in the fixed housing and the seal ring supported by the O-ring installed in the rotational casing make sliding contact with each other. As a result, lubricant inside the crawler-driving apparatus is sealed to prevent an external leakage, and a foreign object is prevented from invading the apparatus from the outside.